1. Field of the Invention
The present invention relates generally to apparatus and methods for taking core samples of subterranean formations. Specifically, the present invention relates to a sponge core barrel assembly, and methods of using the same, for obtaining a formation core sample while maintaining the structural and chemical integrity of the core sample for subsequent analysis.
2. State of the Art
Formation coring is a well-known process in the oil and gas industry. In conventional coring operations, a core barrel assembly is used to cut a cylindrical core from the subterranean formation and to transport the core to the surface for analysis. Analysis of the core can reveal invaluable data concerning subsurface geological formations and, particularly, hydrocarbon-bearing formations, including parameters such as permeability, porosity, and fluid saturation, which are useful in the exploration for petroleum, gas, and minerals. Such data may also be useful for construction site evaluation and in quarrying operations.
A conventional core barrel assembly typically includes an outer barrel assembly, a core bit, and an inner barrel assembly. Generally, a conventional outer barrel assembly comprises one or more hollow cylindrical sections, or “subs,” which are typically secured end-to-end by threads. Secured to a lower end of the outer barrel assembly is the core bit, which is adapted to cut a cylindrical core and to receive the core in a central opening, or throat. The opposing upper end of the outer barrel assembly is attached to the end of a drill string, which conventionally comprises a plurality of tubular sections that extend to the surface. Disposed within the outer barrel assembly is the inner barrel assembly, which is configured to receive the core as the core traverses the throat of the core bit and to retain the core for subsequent transportation to the surface, is the inner barrel assembly.
The outer barrel assembly typically includes a swivel assembly disposed proximate an upper end thereof from which the inner barrel assembly is suspended, an upper end of the inner barrel assembly being releasably secured to the swivel assembly. The swivel assembly includes a thrust bearing or bearings enabling the core bit and outer barrel to rotate freely with respect to the inner barrel assembly suspended within. A conventional outer barrel assembly typically includes a safety joint disposed at its upper end proximate the drill string. If the core barrel assembly becomes wedged or jammed in a bore hole during coring, the safety joint enables the inner barrel assembly and core to be removed, while leaving the outer barrel assembly in the bore hole for subsequent retrieval. The outer barrel assembly may also include one or more sections including core barrel stabilizers that reinforce and stabilize the core barrel during coring, thereby reducing bending of the core barrel assembly and wobble of the core bit. A core barrel assembly may further include an outer tube sub having one or more wear ribs that function to reduce contact between the outer barrel assembly and the wall of the wellbore and, hence, wear of the outer barrel.
Conventional core bits are generally comprised of a bit body having a face surface on one end. The opposing end of the core bit is configured, as by threads, for connection to the lower end of the outer barrel assembly. Located at the center of the face surface is the throat, which extends into a hollow cylindrical cavity formed in the bit body. The face surface includes a plurality of cutters arranged in a selected pattern. The pattern of cutters includes at least one outside gage cutter disposed at the periphery of the face surface that determines the diameter of the bore hole drilled in the formation. The pattern of cutters also includes at least one inside gage cutter disposed adjacent and protruding within the diameter of the throat to determine the outside diameter of the core being cut as it enters the throat.
During coring operations, a drilling fluid is usually circulated through the core barrel assembly to lubricate and cool the plurality of cutters disposed on the face surface of the core bit and to remove formation cuttings from the bit face surface to be transported upwardly to the surface through an annulus defined between the drill string and the wall of the bore hole. A typical drilling fluid, or drilling mud, may include a hydrocarbon or water base or fluid carrier in which fine-grained mineral matter is suspended. The core bit usually includes one or more ports or nozzles positioned to deliver drilling fluid to the face surface. Generally, a port includes a port outlet at the face surface in fluid communication with a bore. The bore extends through the bit body and terminates at a port inlet. Each port inlet is in fluid communication with an annular region defined between the outer barrel assembly and the inner barrel assembly. Drilling fluid received from the drill string under pressure is circulated into the annular region, which enables the port inlet of each port to draw drilling fluid from the annular region. Drilling fluid then flows through each bore and discharges at its associated port outlet to lubricate and cool the plurality of cutters on the face surface and to remove formation cuttings as noted above.
Located within the outer barrel assembly, and releasably attached to the swivel assembly, is the inner barrel assembly. The inner barrel assembly includes an inner tube configured for retaining the core and a core shoe disposed at one end thereof adjacent the throat of the core bit. The core shoe is configured to receive the core as it enters the throat and to guide the core into the inner tube. A core catcher may be disposed proximate the core shoe to assist, in conjunction with the core shoe, in guiding the core into the inner tube and also to retain the core within the inner tube. Thus, as the core is cut—by application of weight to the core bit through the outer barrel assembly and drill string in conjunction with rotation of these components—the core will traverse the throat of the core bit to eventually reach the rotationally stationary core shoe, which accepts the core and guides it into the inner tube where the core is retained until transported to the surface for examination.
Disposed proximate the upper end of the inner barrel assembly where the inner barrel assembly joins to the swivel assembly is a pressure relief plug. The pressure relief plug allows drilling fluid to circulate through the inner tube to flush the inner tube and to clean the bottom of the bore hole prior to coring. To commence coring, a drop ball is seated in the pressure relief plug to divert drilling fluid away from the inner tube and into the annular region between the outer and inner barrels. As the core enters the inner tube, the pressure relief plug also functions to relieve pressure within the inner tube.
The discharge of drilling fluid from the port outlets at the face surface of a core bit during a coring operation may result in drilling fluid invasion of the core. Drilling fluid invasion may result from any one of a number of conditions, or a combination thereof. Drilling fluid discharged at the face surface of the core bit may, if not appropriately directed radially outward away from the core, flow towards the core being cut where the drilling fluid can then contact the core. Also, in most conventional core bits, a narrow annulus exists in a region bounded by the inside diameter of the bit body and the outside diameter of the core shoe, this narrow annulus essentially being an extension of the annular region and terminating at an annular gap proximate the entrance to the core shoe near the throat of the core bit. Pressurized drilling fluid circulating in the annular region may, in addition to flowing into the port inlets, flow into the narrow annulus and out through the annular gap to be discharged proximate the throat of the core bit. This drilling fluid entering the narrow annulus and exiting the annular gap proximate the throat of the core bit, referred to as “flow split,” can contact the core being cut as the core traverses the throat and enters the core shoe. Further, a low rate of penetration (“ROP”) through the formation being cored can lead to drilling fluid invasion of the core as the exposure time of the core to drilling fluids is unduly prolonged.
Drilling fluid invasion can cause a number of deleterious effects, including flushing of reservoir fluids from the core and chemical alteration of the properties of the reservoir fluids. Flushing and chemical alteration of the reservoir fluids in the core can inhibit core analysis and prevent the acquisition of reliable formation data, especially fluid saturation properties such as oil and water saturation. As a result of drilling fluid invasion, it may also be difficult to obtain reliable data for other formation characteristics, such as permeability and wettability.
Another significant factor that may inhibit the acquisition of reliable formation fluid saturation data is reservoir gas expansion resulting from a large pressure differential between the bottom of the bore hole and the surface. As a core sample is raised to the surface from the bottom of the bore hole, where the pressure may be relatively high, gases entrained within the core sample will expand and migrate out of the core sample. The expansion and migration of reservoir gases from the core sample often cause reservoir fluids contained within the core sample to be expelled. The expelled reservoir fluids are difficult, if not impossible, to recover and, therefore, the reliable measurement of fluid saturation properties is impeded.
One conventional approach to preserving the integrity of the core and obtaining reliable formation data, especially reservoir fluid properties such as oil and water saturation, is sponge coring. Sponge coring is performed using a “sponge core barrel.” Generally, a sponge core barrel comprises a conventional core barrel assembly, as was described above, that has been adapted for use with a plurality of sponge liners. Each sponge liner includes a layer of absorbent material selected for its ability to absorb the reservoir fluid of interest (for example, oil) from a core sample.
A conventional sponge liner comprises an annular sponge layer encased in a tubular sleeve. The annular sponge layer is constructed of a material adapted to absorb a specified reservoir fluid of interest. For example, if the particular formation characteristic of interest is oil saturation, the sponge layer is constructed of an oil-absorptive material such as polyurethane. To obtain formation water saturation data, a water-absorptive material is used to construct the sponge layer. A common water-absorptive material used for the construction of the sponge layer is a cellulose fiber and polyurethane composite.
The tubular sleeve provides structural support for the annular sponge layer and is typically constructed of a relatively rigid material such as aluminum. The annular sponge layer is adhered to the interior cylindrical surface of the sleeve, which may include a plurality of ribs extending radially inward therefrom. The ribs provide additional structural support for the sponge layer and also provide additional surface area to which the sponge layer may adhere. However, even with the addition of radially extending ribs, the annular sponge layer may separate or peel away from the surfaces of the ribs and the cylindrical interior of the tubular sleeve during coring. Also, the tubular sleeve may include a plurality of holes or other perforations to compensate for expansion of formation gases, as will be described below.
The inner barrel assembly of a sponge core barrel includes an inner tube adapted to receive the plurality of sponge liners, the inner diameter of the inner tube being substantially equal to the outer diameter of a sponge liner. During a coring operation, a core shoe disposed at the lower end of the inner tube guides the core being cut into the inner tube and sponge liners disposed therein, where the core is retained for subsequent transportation to the surface and later analysis. The cylindrical interior cavity of the annular sponge layer is of a diameter substantially equal to the diameter of the core being cut, such that the interior cylindrical surface of the annular sponge layer substantially continuously contacts the exterior surface of the core. The substantially continuous contact between the annular sponge layer and the core often results in the application of significant frictional forces on the core.
When the inner barrel assembly and core are raised to the surface, where the ambient pressure may be significantly less than the downhole pressure, formation gases within the core sample may expand and expel reservoir fluids from the core. The expelled reservoir fluids are then absorbed by the annular sponge layer and preserved for later analysis, rather than separating from the core sample and flowing out, as by gravity, from the inner tube. The perforations in the sleeve of the sponge liner allow reservoir gases to escape. Also, because the sponge layer contacts the core and is relatively flexible as compared to the core, the sponge liners serve to contain the core and protect the core from mechanical damage.
Sponge liners are typically supplied in standard 5-ft or 6-ft sections, a number of which are placed end-to-end within the inner tube to substantially fill the length, (usually a standard 30 feet) of the inner tube. The inner tube is typically constructed of a steel material and, as indicated above, the tubular sleeve of a conventional sponge liner comprises an aluminum material. Due to the differences in material properties of the tubular sleeve and the inner tube, the coefficient of thermal expansion for aluminum is approximately twice that of steel, and the long extent of the inner tube and sponge liners disposed end-to-end therein, the conventional sponge core barrel assembly routinely experiences differential thermal expansion. Differential thermal expansion between the inner tube and sponge liners may occur longitudinally along the length of the inner tube as well as radially. Differential thermal expansion may cause mechanical damage to components of the sponge core barrel assembly and may also damage the core sample.
Differential thermal expansion between the inner barrel assembly and the outer barrel assembly may also be present. The various components making up the outer barrel assembly are usually constructed of one or more types of alloy steel. Although the inner tube sections are typically constructed of a steel material, as noted above, it may be desirable to construct the inner tube sections from other suitable materials, such as aluminum and composite materials. If the outer barrel assembly and inner barrel assembly are constructed of materials exhibiting significantly different thermal expansion characteristics, differential thermal expansion between the outer and inner barrel assemblies will result. Differential thermal expansion between the outer barrel assembly and the inner barrel assembly can cause a number of problems during coring. Specifically, such differential thermal expansion can cause mechanical damage to the core barrel and may result in additional drilling fluid invasion due to increased flow split.
As noted above, flow split is the result of the flow of drilling fluid from the annular region between the inner and outer barrel assemblies and through a narrow annulus that exists between the bit body and the core shoe, to be exhausted through an annular gap near the throat of the core bit and proximate the core sample. The annular gap is defined by a longitudinal distance between the lower end of the core shoe and the bit body. The width of the annular gap—and, hence, the volume of flow split—is a function of the difference between the longitudinal length of the outer barrel assembly and the longitudinal length of the inner barrel assembly; the inner barrel assembly being suspended at its upper end from a swivel assembly disposed proximate the upper end of the outer barrel assembly. Although the provision of a narrow annulus and annular gap may result in flow split, the narrow annulus and annular gap are necessary as the clearance between the core shoe and the bit body provided by the narrow annulus and annular gap enables the outer barrel assembly and core bit to rotate freely relative to the inner barrel assembly. Thus, it is desirable to maintain the width of the annular gap at a controlled, minimum distance.
Conventionally, in order to maintain the width of the annular gap at a specified value in lieu of differential thermal expansion between the inner and outer barrel assemblies, the magnitude of the differential thermal expansion is calculated based on an estimated or known downhole temperature and an adjustment is made based on this calculated value. Typically, the adjustment comprises leaving a large spacing between the end of the inner barrel assembly (i.e., the core shoe) and the lower end of the outer barrel assembly (i.e., the bit body), the large spacing being closed by differential thermal expansion between the inner and outer barrel assemblies. However, this method of compensating for differential thermal expansion between the inner and outer barrel assemblies is prone to human error and is susceptible to unexpected downhole temperature swings.
In conventional sponge coring operations, in order to protect the sponge liners from drilling fluid contamination prior to commencement of coring and from being compressed as a result of high downhole pressure, the inner tube is evacuated and filled with a presaturation fluid. The presaturation fluid is selected such that it will not be absorbed by the annular sponge layer, i.e., the presaturation fluid comprises a base fluid that exhibits characteristics opposite to those of the reservoir fluid being measured. For example, if oil saturation data is required, the presaturation fluid may include water as the base fluid. Presaturation usually occurs on the floor of the drilling rig after an inner barrel is assembled. A valve disposed at the upper end of the inner tube enables the evacuation of the inner tube and the subsequent pumping of presaturation fluid into the inner tube.
Containment of the presaturation fluid within the inner tube prior to entry of the core is provided by a sealing mechanism disposed at the lower end of the inner tube proximate the core bit. The sealing mechanism must be capable of retaining the presaturation fluid under pressure within the inner tube prior to commencement of coring and, further, must enable the presaturation fluid to flow out of the inner tube upon entry of the core into the inner tube. The sealing mechanism also prevents the entry of drilling fluid into the inner tube from the throat of the core bit. A number of sealing mechanisms for use in sponge coring operations are known in the art.
Disclosed in U.S. Pat. No. 4,598,777 to Park et al. is a piston seal assembly comprising a piston disposed at the lower end of an inner tube and an O-ring providing a fluid seal between the piston and the interior wall of the inner tube. Prior to coring, the piston remains at the lower end of the inner tube to retain the presaturation fluid within the inner tube and to prevent ingress of drilling fluids into the inner tube. When coring begins, the core traverses the throat of the core bit and contacts the lower end of the piston, dislodging the piston and pushing the piston upwardly into the inner tube. As the piston begins to move upwardly, the fluid seal provided by the O-ring is broken, allowing presaturation fluid to flow around the piston and out through the lower end of the inner tube and the throat of the core bit. Due to thermal expansion of the presaturation fluid and to compression of the sponge core barrel resulting from high downhole pressure, the presaturation fluid within the inner tube may exhibit a high pressure prior to coring. To break the fluid seal and dislodge the piston, the core must overcome forces resulting from this high pressure, as well as any frictional forces generated between the O-ring and the interior wall of the inner tube. Large compressive forces may be applied to the end of the core in overcoming the high pressure exerted on the piston and any frictional forces, which may cause structural damage to the core.
U.S. Pat. No. 4,479,557 to Park et al. discloses a seal mechanism comprising a diaphragm and a piercer. The diaphragm comprises a rupturable membrane positioned at the lower end of the inner tube that, prior to being ruptured, is capable of retaining presaturation fluid within the inner tube and inhibiting the flow of drilling fluid thereinto. The piercer comprises a piston movable through the inner tube having a lower, planar end configured for contacting the core and an opposing, conical end configured for piercing the diaphragm. As a core is cut and enters the throat of the core bit, the core contacts the lower end of the piercer and pushes the piercer upwardly through the inner tube. The apex of the piercer then contacts and ruptures the diaphragm, enabling some presaturation fluid to flow out around the piercer while the remainder of the presaturation fluid is forced out through a check valve at the upper end of the inner tube as the piercer and core traverse the inner tube. Again, however, the presaturation fluid may be subject to high pressure prior to the commencement of coring and, as a result, high compressive forces may be exerted on the core during rupturing of the diaphragm.
As suggested above, a conventional assembled sponge core barrel comprises a standard 30-ft outer barrel assembly having a core bit secured to a lower end thereof. Disposed within the outer barrel assembly, and rotationally suspended from a swivel assembly, is a standard 30-ft inner barrel assembly. The inner barrel assembly includes an inner tube with a plurality of 5-ft or 6-ft sponge liners disposed end-to-end therein. The inner barrel is assembled on the drilling rig floor and is subsequently evacuated and filled with presaturation fluid prior to being picked up and lowered into the outer barrel assembly, which is suspended from the rig floor. Use of a 30-ft sponge core barrel assembly, however, inherently limits the efficiency of sponge coring operations. The sponge core barrel assembly must be raised from the bore hole when the maximum length of core has been retrieved inside the inner barrel, such that the core sample can be removed from the inner barrel assembly and new sponge liners inserted. Raising, or tripping, of a drill string from the bore hole is a time-consuming operation and, therefore, it is desirable to core with core barrels greater than 30 feet in length.
Conventional coring operations—not including conventional sponge coring—are routinely performed using core barrel lengths of 60 feet, 90 feet, 120 feet, or longer. Make up of the outer barrel assembly typically comprises interconnecting the various components of the outer barrel assembly while suspending the outer barrel through the floor of the drilling rig. In other words, each component of the outer barrel assembly is individually or, in conjunction with other attached components, lifted off the rig floor and secured to the partially assembled outer barrel (i.e., those components already assembled), which is suspended from the rig floor. Subsequently, the inner barrel assembly is rigged up section-by-section within the outer barrel assembly, interconnections between the inner barrel sections being made just above the upper end of the outer barrel assembly. The inner barrel assembly is then secured to a swivel assembly that is attached to the outer barrel assembly, the swivel assembly rotationally isolating the inner barrel assembly from the outer barrel assembly.
By way of example, 90-ft outer barrel assembly having a core bit secured to a lower end thereof may be rigged up and suspended through the rig floor. A first 30-ft section of inner barrel having a core shoe at a lower end thereof is then lowered into the outer barrel assembly, a portion of the upper end of the first inner barrel section extending above the outer barrel assembly. Next, a second 30-ft section of inner barrel is lifted off the rig floor and a lower end thereof is connected to the upper end of the first inner barrel section, the first and second inner barrel sections then being lowered into the outer barrel assembly with a portion of the upper end of the second inner barrel section extending above the outer barrel assembly. A third 30-ft section of inner barrel is then lifted off the rig floor and a lower end of this third section is connected to the upper end of the second inner barrel section. The first, second, and third interconnected inner barrel sections are then lowered into the outer barrel assembly. Additional components may be secured to the upper end of the third inner barrel section, such as a pressure relief plug and drop ball. The first, second, and third inner barrel sections—the inner barrel assembly—is then secured to a swivel assembly that is attached to the outer barrel assembly. The upper end of the outer barrel assembly is subsequently secured to the lower end of a drill string for coring.
During make up of the inner barrel assembly, a section of inner tube, or two or more interconnected inner tube sections, may be stored in a mouse hole prior to being hoisted above the outer barrel assembly for assembly and insertion thereinto. A mouse hole is an opening extending through and below the rig floor into which one or more inner tube sections (as well as outer barrel components) may be temporarily placed for make up and subsequent transfer to the outer barrel assembly. Offshore drilling rigs commonly have a mouse hole extending to a depth of 60 feet or more below the rig floor.
It would be desirable to conduct sponge coring operations with a core barrel assembly greater than 30 feet in length, i.e., using a 60-ft, 90-ft, 120-ft or other desired extended-length core barrel comprised of multiple 30-ft (or some other suitable length) sections of inner barrel, such as is routinely performed in conventional coring operations, as noted above. However, to present day, it has been thought impossible to conduct sponge coring operations with extended-length core barrels, i.e., one having a length greater than 30-feet, due to a number of technical difficulties. Specifically, frictional forces generated between a core and a sponge-lined inner barrel increase as a function of length of the sponge-lined inner barrel, and high frictional forces can adversely affect the mechanical integrity of the core, as well as cause damage to the sponge material. Thus, for sponge-lined inner barrels longer than the conventional 30 feet, it has been believed that, without significant improvements of the sponge material, extreme frictional forces would be generated between the sponge materials, such extreme frictional forces leading to core damage and structural failure of the sponge material. Also, differential thermal expansion and resultant problems, as noted above, become more pronounced with increasing length of the core barrel assembly. Further, suitable methods and apparatus for performing sponge coring with extended-length core barrels are presently unavailable. For example, methods and apparatus for separately presaturating and subsequently interconnecting individual sections of inner tube were heretofore unknown.
Thus, a need exists in the art of subterranean formation coring for apparatus and methods for performing sponge coring that overcome the limitations of the prior art. Specifically, a need exists for a sponge core barrel assembly having an inner barrel assembly adapted to control the presaturation fluid pressure and further including an easily actuated sealing mechanism, such that damage to the core during depressurization and release of the presaturation fluid is eliminated. A need also exists for a sponge core barrel assembly comprised of multiple inner barrel sections and having a length greater than the conventional 30 feet. Yet another need exists for a sponge core barrel assembly adapted to compensate for differential thermal expansion between the inner tube and one or more sponge liners, as well as adapted to compensate for differential thermal expansion between the outer barrel assembly and the inner barrel assembly. Further, a need exists for a high-strength sponge liner resistant to debonding of the sponge layer from the surrounding sleeve, and a need exists for such a sponge liner that imparts minimal frictional forces to the core.